Microstructure evolution in thin zirconia films: Experimental observation and modelling

The gas-tightness and high ionic conductivity of the electrolyte are essential requirements for realization of high efficiency and stable operation of solid oxide fuel cells. In this study, the microstructure of yttria-stabilized zirconia electrolyte films sintered on a rigid substrate was characterized using scanning electron microscopy (SEM). The pore size, shape and orientation were quantitatively determined based on image analysis of SEM micrographs. It was found that the pores are elongated and oriented preferentially perpendicular to the film plane. The average pore size was found to increase, while the pore elongation and preferential orientation decrease with sintering. A variational principle-based sintering model capable of predicting microstructural evolution is presented. The modelling results agree with the experimental observations in that constrained sintering leads to an anisotropic microstructure which becomes more isotropic with densification. Decreasing grain boundary diffusion with respect to surface diffusion is predicted to produce a more isotropic pore geometry. The model also predicts that in constrained sintering there exists a critical pore size above which the pore grows instead of shrinks. It is critical to control defect size at an early stage of processing to be smaller than the critical pore size in order to obtain leak-free electrolyte films.